I know there is a "feedback" from the PA to the PDM board I believe it is, so after readjusting the power pots this is what I have now:

All multi meter readings pretty much normal. Low PA current. Low power out. I can get a KW out of it before things start to go south.

Here is something that really stood out to me:

When adjusting the power pots to increase power (starting from zero watts out), plate current goes up, but Grid current goes down. It gets to a point where grid current is 0, and that is about all she wrote. Any further attempt and increasing power only increases PA voltage. Power out doesn't change.

I am just not intimately familiar with this transmitter. If anyone can at least give me a clue I would greatly appreciate it!

I am a tad younger than most engineers I know, so I lack the "tube" experience. All my electronics training/schooling was solid state.

I normally change the tubes in this transmitter about every 2 years. I don't just change them on a time line; I normally wait until positive modulation starts falling off, which is normally pretty gradually. I have noticed in the last couple months it has been falling a little bit, but nothing to write home about. I have never seen a set of tubes fall flat on their face so quickly.

Anyway, I went back out there and ran it at 2kw. As I started to increase the power, positive modulation fell flat, and the grid current went down. When I hit that point, I reached down and cranked the filament control to max (over volted them). Power came up, grid current came up, and my positive modulation came back as well.

On the other hand, it seems odd that you'd go from 10kW to less than 2 without noticing it on the way. That's a sudden drop to lay at the feet of an aging tube.

This might seem simplistic, but considering you're getting such a dramatic change by raising the filament voltage... are you sure there's not a problem there?

Numerous times, I've read posts from other engineers, advising resistance to blame sudden problems on what seems the most obvious... the tube(s). Thinking back over 50 years of working with various rigs, I've had shorts crop up in a very few power tubes, but overall, they've been far more reliable than one might expect.

I would agree. Another tip that I picked up from somewhere (can't remember now), was it is almost NEVER the tubes. That is also why I was second guessing myself. Even though not long ago I had a filament open up in a 6 month old tube in a BE FM25T LOL.

But in this case...oh, how do I say this in a politically correct fashion:

There are no "daily log readings" to track the recent history of the transmitter performance. Last time I was at the site was a month ago, and things were fine then. As I said I had been noticing some loss of positive modulation the last couple months, but nothing earth shattering.

But I agree, that is a short period of time for it to fall flat like that.

I've worked on MW-5 in the past and ran across similar symptoms when a some resistors in the DC supply opened up. IIRC these were power resistors that were located near the front of the rig and were inside on the swing open door. It's been many years since, so I may be imagining things

I am putting my two cents in here, and probably oing out on a limb at the same time.

"Back in the day" I had an MW-5 go down. Don't recall exact symptoms, but might have been similar.

It acted like the solid state driver had failed. No stock on the shelf in Quinky at the time. Traced things out and decided to use a "remembered trick" with a clip lead to bring up drive thru pdm. This was after thorough check of fuse continuity.. Placed clip lead in the "proper places" and heard a slight "POP". Blew fuse as I forgot they changed the connection sequence. Replaced fuse and rig came to life at full power.

What had happened was that the fuse had developed a resistive path connection in the end cap that would show continuity and voltage, but would not pass enough current to light up the solid state driver module. Second time I got caught in that same trap. First one was in a fan circuit in a car. Duhhhhh!

It WAS just the tubes. They came in yesterday, and it tuned right up. Thanks again to everyone who chimed in though. There may come a time when I get stuck again.

This thing has lived a hard life. And it does seem to break down I swear once a month. I have been pretty lucky so far in I have been able to tackle most of its issues, but every once in a while I end up scratching my head...

I know this isn't really on topic, but maybe it will help you or someone else later...

A client replaced a MW-10 with a Nautel XR-12 awhile ago thinking that it would pay for itself in about 5-6 years with savings on the electric bill. Between the electrical savings, the elimination of tube replacement costs, the lower heat load in the transmitter building and almost $2000/mo in repair costs, it ended up being about three.

I don't do a whole lot of contract work. But most of the ones I do work for are very penny-wise, but dollar foolish. They would rather spend 1k a month maintaining something, than spend 10k all at once to replace it with something more reliable, even when you present them with the logic.

The other issue at this particular location is I don't think a new solid state transmitter would survive the environment this old MW10 operates in. Yes, the facilities are in that bad a state of dis-repair. I have voiced my opinion about it, but I guess they will just keep doing what they are doing until the towers fall over or building collapses, then they will deal with it, or just close shop.